Long term evolution standardization in Third Generation Partnership Project (which may be referred to as “3GPP LTE”) Release 8 to Release 11 have adopted horizontal beamforming using linear array antenna elements arranged in the lateral dimension.
In 3GPP LTE Release 12, a 3D-MIMO beamforming scheme that uses a number of antenna elements arranged two-dimensionally in the vertical and the lateral directions is being discussed. This new technology allows a base station to perform vertical beamforming in addition to horizontal beamforming. See, for example, non-Patent documents 1 and 2 listed below. By providing directionality in both the vertical (or elevation) and the horizontal (or azimuth) directions, improvement of system characteristics is expected.
Up to the 3GPP LTE Release 11, closed loop MIMO precoding has been achieved by returning channel state information (CSI) in the horizontal direction and between polarized antenna elements back to a MIMO base station. In order to reduce the CSI overhead, a codebook describing a set of precoding matrices (linear filters) is shared between the base station and user equipment. The user equipment selects a desired precoding matrix from the codebook and reports the selected matrix index together with the channel quality indicator (CQI) to the base station. The base station applies the precoding matrix to transmission data based upon the feedback information supplied from the user equipment.
For downlink MIMO transmission, codebooks designed for 8 transmit antennas have come to agreement. See, for example, non-patent Document 3 listed below.